1. Field of the Invention
The present invention relates to a brake service signaling warning system for a vehicle brake and more specifically to a brake lining wear signaling system for an xe2x80x9cSxe2x80x9d cam type truck brake where a switch is mounted on the actuator lever arm.
2. Description of the Prior Art
The braking systems in transport trucks have parts that wear and are expendable and by reason of this such braking systems require continuous surveillance for proper maintenance. Unfortunately many braking systems, and especially the portions that are expendable, are concealed or difficult to perform inspection and some require at least partial dismantling to learn of their condition. Inspection procedures are inconvenient and time consuming and consequently brake system maintenance is often neglected which often leads to substantial safety hazards.
By far the greatest number of transport vehicle accidents that are caused by vehicular equipment are caused in some fashion by braking systems. Responsively, various sensing systems have heretofore become known to sense and sometimes annunciate brake condition, but no such systems have effectively resolved all of the associated problems. The instant invention provides a new, novel and improved member of this class of device.
Almost universally truck braking systems have standardized drum-type brakes with two arcuate shoe arms pivotally carried at first adjacent ends inside a brake drum for expansion against the inner surface of the drum to cause frictional braking action. The shoe arms are expanded by an xe2x80x9cSxe2x80x9d cam that is rotated between second adjacent ends of the opposed arms to cause both shoe arms to move radially outwardly in substantially similar fashion.
Most modern pneumatic braking systems have a diaphragm type canister that applies motive force to an actuator arm or lever which can include a slack adjuster mechanism that in turn rotates the xe2x80x9cSxe2x80x9d cam shaft for braking. A valve operated by the vehicle brake pedal releases compressed air to the canister to move the diaphragm. By the nature of this system, unlike hydraulic systems common in automobiles, little if any indication of brake condition is given to an operator. The pneumatic canister that applies braking force is limited in its stroke and because of wear of the brake shoes, can progress to the point that the canister stroke is not enough to apply adequate force to the brakes. If a slack adjuster is available, then the actuator lever can be repositioned and the brakes can be used until the brake shoes wear out. Because of the high mechanical advantage built into the system to provide larger motion of the air canister diaphragm that is translated to a smaller motion of the brake shoes, the wear condition is not well indicated to an operator by brake function.
Various devices have heretofore become known to sense motion parameters of braking systems to determine system condition. The most common parameter measured has been the movement of a cylinder or diaphragm which ultimately moves the brake shoe arms (see U.S. Pat. Nos. 5,226,509 and 5,213,056 the disclosures of which are hereby incorporated by reference. Other devices have sensed the thickness of brake shoe lining, commonly by signally when the distance between a brake shoe arm and the associated brake drum reaches a given point (see U.S. Pat. Nos. 3,533,491 and 3,958,445). At least one prior device as disclosed in U.S. Pat. No. 5,253,735, the disclosure of which is hereby incorporated by reference, senses the angular motion of the slack adjuster arm that rotates the xe2x80x9cSxe2x80x9d cam shaft to indicate brake shoe lining wear. Some of these sensing devices have proven to be effective at signaling the point that brake maintenance is required, but various limitations have prevented wide spread use in the truck industry.
Most known brake condition sensors have been concerned with the state of brake shoe linings, though the amount of xe2x80x9cthrowxe2x80x9d or motion required to actuate the braking system is generally of substantially equal importance. xe2x80x9cSxe2x80x9d cam activated braking systems generally may be adjusted by a slack adjustment mechanism incorporated into the lever which changes the angular position of the xe2x80x9cSxe2x80x9d cam shaft at which rotary motion is first applied to move that shaft. This allows a brake system to be adjusted to accommodate wear of both the brake shoe lining and of the xe2x80x9cSxe2x80x9d cam and its contacting pins, thereby staying within the limits of motion of the actuator and linkage. If slack adjustment is not properly made, the air canister rod stroke may not be sufficient to properly operate the braking system and if shoe wear is great the xe2x80x9cSxe2x80x9d cam can even rotate off brake shoe contact pins to cause a braking system to become inoperative.
Slack adjusters incorporated into the actuator lever are either manually or automatically adjustable to compensate for wear of the brake lining, such adjustment being usually accomplished by rotating the cam shaft relative to the slack adjuster lever to reposition the cam between the brake shoe ends so that in the brake relaxed condition, the clearance distance between the brake shoe linings and the brake drum can be maintained at a given distance regardless of lining wear.
After the actuator lever or actuator lever with slack adjuster reaches its limit of travel, it would be desirable to signal the end of service life of the brake lining to the operator so that new linings can be installed. The rotational position of the cam shaft and the actuator lever is therefore an indication of the degree of wear of the brake linings.
If no automatic slack adjuster is provided, then the operator or mechanic must manually adjust the length of the link connecting the actuator to the lever to provide proper actuation of the brake. Once the brake lining has worn to its service limit, then the brake shoes must be relined or replaced. Unless the service limit of the brake lining is somehow indicated by reference to the lever or the link geometry, then the brake drum must be removed to allow for visual inspection of the linings and the position of the xe2x80x9cSxe2x80x9d cam. The xe2x80x9cSxe2x80x9d cam can also reach a service limit where any additional rotation of the cam shaft would result in the xe2x80x9cSxe2x80x9d cam rotating into a position where brake shoes expansion no linger occurs and the xe2x80x9cSxe2x80x9d cam and brake can sustain damage.
Thus, it would be desirable to give the operator an indication that the brakes have reached or are about to reach their service limit so that the proper maintenance can be undertaken prior to degradation of brake performance and/or damage.
The present invention provides a system to monitor the travel of the actuator and actuator lever and subsequent rotation of the cam shaft to provide a signal to the vehicle operator when the brake linings require servicing. A modification is made to the brake rod clevis to allow for the mounting of a push switch positioned to be activated by contact with the actuator lever (which may incorporate a slack adjuster) when the brake service limit has been reached and/or when the slack in the brake actuation mechanism requires adjustment. A mounting boss formed into the clevis casting provides a mounting structure for the switch while a qualified surface machined into the actuation lever provides a precise angular point of contact to be made with the switch. In this manner, as the brake lining wears, the angular relationship between the actuator rod and the actuator lever changes to reflect that wear and the switch mounted to the clevis contacts and is actuated by contact with the brake lever at a given point of brake lining wear and/or to signal when manual adjustment of the brake slack adjuster is required.
An object of the present invention is to sense when the relative angle between the brake actuator rod and the actuator lever has decreased to a given point thereby indicating that service of the brake system is required.
Another object of the present invention is to sense the relative angle between the brake actuator rod and the actuator lever using a switch mounted to either the actuator rod or the actuator lever.
Another object of the present invention is to sense the relative angle between the brake actuator rod and the actuator lever using a switch mounted to the clevis link of the actuator rod and positioned to contact the actuator lever.
Still another object of the present invention is to sense the relative angle between the brake actuator rod and the actuator lever as a means to indicate to the operator that brake service is required.